History

Early forms of distillation were known to
Babylonian
alchemists in
Mesopotamia (in what is now
Iraq)
from at least the
2nd millennium BC.[1]
Archaeological excavations in northwest
Pakistan, have yielded evidence that the distillation of
alcohol was known in
South Asia since 500 BCE.[2]
But only became common between 150 BCE- 350 CE.[2]
Distillation was later known to
Greek alchemists from the
1st century AD,[3][4][5]
and the later development of large-scale distillation apparatus
occurred in response to demands for spirits.[3]
Hypathia of Alexandria is credited with having invented an
early distillation apparatus,[6]
and the first exact description of apparatus for distillation is
given by Zosimos of
Alexandria in the fourth century.[5]
Primitive tribes of India used a method of distillation for
producing Mahuda liquor. This crude and ancient method is
not very effective.[7]

In the 8th century, alchemists in the Middle East produced
distillation processes to purify
chemical substances for
industrial purposes such as isolating natural
esters (perfumes)
and producing pure
alcohol.[8]
The first among them was the
PersianJabir
ibn Hayyan (Geber) circa 800 AD, who is credited with
the invention of numerous chemical apparatus and processes that
are still in use today. In particular, his
alembic was the first
still
with
retorts which could fully purify chemicals, a precursor to
the
pot still, and its design has served as inspiration for
modern micro-scale distillation apparatus such as the Hickman
stillhead.[9]
Petroleum was first distilled by another
Persian,
al-Razi (Rhazes) in the 9th century, for producing
kerosene,[10]
while
steam distillation was invented by
Avicenna in the early 11th century, for producing
essential oils.[11]
As the works of Middle Eastern scribes made their way to India
and became a part of Indian alchemy, several texts dedicated to
distillation made their way to Indian libraries.[12]
Among these was a treatise written by a scholar from Bagdad in
1034 titled Ainu-s-Sana'ah wa' Auna-s-Sana'ah.[12]
Scholar Al-Jawbari travelled to India.[13]
By the time of the writing of the
Ain-e-Akbari, the process of distillation was well known
in India.[14]

In
1500,
German alchemist Hieronymus Braunschweig published Liber
de arte destillandi (The Book of the Art of Distillation)[1]
the first book on the subject, followed in
1512
by a much expanded version.

In 1651,
John French published
The Art of Distillation the first major English compendium
of practice, though it has been claimed[15]
that much of it derives from Braunschweig's work. This includes
diagrams with people in them showing the industrial rather than
bench scale of the operation.

As
alchemy evolved into the science of
chemistry, vessels called
retorts became used for distillations. Both alembics and
retorts are forms of
glassware with long necks pointing to the side at a downward
angle which acted as air-cooled
condensers to
condense the distillate and let it drip downward for
collection.

Later, copper alembics were invented. Riveted joints were
often kept tight by using various mixtures, for instance a dough
made of rye flour.[16]
These alembics often featured a cooling system around the beak,
using cold water for instance, which made the condensation of
alcohol more efficient. These were called
pot stills.

Today, the retorts and pot stills have been largely
supplanted by more efficient distillation methods in most
industrial processes. However, the pot still is still widely
used for the elaboration of some fine alcohols such as
cognac,
Scotch whisky and some
vodkas. Pot stills made of various materials (wood, clay,
stainless steel) are also used by
bootleggers in various countries. Small pot stills are also
sold for the domestic production[17]
of flower water or
essential oils.

In the early 19th century the basics of modern techniques
including pre-heating and reflux were developed, particularly by
the French[18],
then in
1830 a British
Patent was issued to
Aeneas Coffey for a whiskey distillation column[19],
which worked continuously and may be regarded as the
archetype of modern petrochemical units. In
1877,
Ernest Solvay was granted a U.S. Patent for a tray column
for
ammonia distillation[20]
and the same and subsequent years saw developments of this theme
for oil and spirits.

The main difference between laboratory scale distillation and
industrial distillation is that laboratory scale distillation is
often performed batch-wise, whereas industrial distillation
often occurs continuously. In
batch distillation, the composition of the source material,
the vapors of the distilling compounds and the distillate change
during the distillation. In batch distillation, a still is
charged (supplied) with a batch of feed mixture, which is then
separated into its component fractions which are collected
sequentially from most volatile to less volatile, with the
bottoms (remaining least or non-volatile fraction) removed at
the end. The still can then be recharged and the process
repeated.

In
continuous distillation, the source materials, vapors and
distillate are kept at a constant composition by carefully
replenishing the source material and removing fractions from
both vapor and liquid in the system. This results in a better
control of the separation process.

Idealized distillation model

The
boiling point of a liquid is the temperature at which the
vapor pressure of the liquid equals the pressure surrounding
the liquid. The
normal boiling point of a liquid is the special case at
which the vapor pressure of the liquid equals the ambient
atmospheric pressure. A liquid in a container at a pressure
below atmospheric pressure will boil at temperature lower than
the normal boiling point, and a liquid in a container at a
pressure higher than atmospheric pressure will boil at a
temperature higher than the normal boiling point.

It is a common misconception that in a liquid mixture at a
given pressure, each component boils at the boiling point
corresponding to the given pressure and the vapors of each
component will collect separately and purely. This, however,
does not occur even in an idealized system. Idealized models of
distillation are essentially governed by
Raoult's law and
Dalton's law.

Raoult's law assumes that a component contributes to the
total
vapor pressure of the mixture in proportion to its
percentage of the mixture and its vapor pressure when pure. If
one component changes another component's vapor pressure, or if
the volatility of a component is dependent on its percentage in
the mixture, the law will fail.

Dalton's law states that the total vapor pressure is the sum
of the vapor pressures of each individual component in the
mixture. When a multi-component liquid is heated, the vapor
pressure of each component will rise, thus causing the total
vapor pressure to rise. When the total vapor pressure reaches
the pressure surrounding the liquid,
boiling occurs and liquid turns to gas throughout the bulk
of the liquid. Note that a given mixture has one boiling point
at a given pressure, when the components are mutually soluble.

The idealized model is accurate in the case of chemically
similar liquids, such as
benzene and
toluene. In other cases, severe deviations from Raoult's law
and Dalton's law are observed, most famously in the mixture of
ethanol and water. These compounds, when heated together,
form an
azeotrope, in which the boiling temperature of the mixture
is lower than the boiling temperature of each separate liquid.
Virtually all liquids, when mixed and heated, will display
azeotropic behaviour. Although there are
computational methods that can be used to estimate the
behavior of a mixture of arbitrary components, the only way to
obtain accurate
vapor-liquid equilibrium data is by measurement.

It is not possible to completely purify a mixture of
components by distillation, as this would require each component
in the mixture to have a zero
partial pressure. If ultra-pure products are the goal, then
further
chemical separation must be applied.

Batch distillation

Heating an ideal mixture of two volatile substances A and B
(with A having the higher volatility, or lower boiling point) in
a batch distillation setup (such as in an apparatus depicted in
the opening figure) until the mixture is boiling results in a
vapor above the liquid which contains a mixture of A and B. The
ratio between A and B in the vapor will be different from the
ratio in the liquid: the ratio in the liquid will be determined
by how the original mixture was prepared, while the ratio in the
vapor will be enriched in the more volatile compound, A (due to
Raoult's Law, see above). The vapor goes through the condenser
and is removed from the system. This in turn means that the
ratio of compounds in the remaining liquid is now different from
the initial ratio (i.e. more enriched in B than the starting
liquid).

The result is that the ratio in the liquid mixture is
changing, becoming richer in component B. This causes the
boiling point of the mixture to rise, which in turn results in a
rise in the temperature in the vapor, which results in a
changing ratio of A : B in the gas phase (as distillation
continues, there is an increasing proportion of B in the gas
phase). This results in a slowly changing ratio A : B in the
distillate.

If the difference in vapor pressure between the two
components A and B is large (generally expressed as the
difference in boiling points), the mixture in the beginning of
the distillation is highly enriched in component A, and when
component A has distilled off, the boiling liquid is enriched in
component B.

Continuous distillation

Continuous distillation is an ongoing distillation in which a
liquid mixture is continuously (without interruption) fed into
the process and separated fractions are removed continuously as
output streams as time passes during the operation. Continuous
distillation produces at least two output fractions, including
at least one
volatile distillate fraction, which has boiled and been
separately captured as a vapor condensed to a liquid. There is
always a bottoms (or residue) fraction, which is the least
volatile residue that has not been separately captured as a
condensed vapor.

General improvements

Both batch and continuous distillations can be improved by
making use of a
fractionating column on top of the distillation flask. The
column improves separation by providing a larger surface area
for the vapor and condensate to come into contact. This helps it
remain at equilibrium for as long as possible. The column can
even consist of small subsystems ('trays' or 'dishes') which all
contain an enriched, boiling liquid mixture, all with their own
vapor-liquid equilibrium.

There are differences between laboratory-scale and
industrial-scale fractionating columns, but the principles are
the same. Examples of laboratory-scale fractionating columns (in
increasing efficacy) include:

Laboratory scale distillation

Laboratory scale distillations are almost exclusively run as
batch distillations. The device used in distillation, sometimes
referred to as a still,
consists at a minimum of a reboiler or pot in
which the source material is heated, a condenser in which
the heated
vapour
is cooled back to the liquid
state, and a receiver in which the concentrated or
purified liquid, called the distillate, is collected.
Several laboratory scale techniques for distillation exist (see
also
distillation types).

Simple distillation

In simple distillation, all the hot vapors produced
are immediately channeled into a condenser which cools and
condenses the vapors. Therefore, the distillate will not be pure
- its composition will be identical to the composition of the
vapors at the given temperature and pressure, and can be
computed from
Raoult's law.

As a result, simple distillation is usually used only to
separate liquids whose boiling points differ greatly (rule of
thumb is 25 °C),[21]
or to separate liquids from involatile solids or oils. For these
cases, the vapor pressures of the components are usually
sufficiently different that Raoult's law may be neglected due to
the insignificant contribution of the less volatile component.
In this case, the distillate may be sufficiently pure for its
intended purpose.

Fractional distillation

For many cases, the boiling points of the components in the
mixture will be sufficiently close that Raoult's law must be
taken into consideration. Therefore, fractional distillation
must be used in order to separate the components well by
repeated vaporization-condensation cycles within a packed
fractionating column.

As the solution to be purified is heated, its vapors rise to
the
fractionating column. As it rises, it cools, condensing on
the condenser walls and the surfaces of the packing material.
Here, the condensate continues to be heated by the rising hot
vapors; it vaporizes once more. However, the composition of the
fresh vapors are determined once again by Raoult's law. Each
vaporization-condensation cycle (called a
theoretical plate) will yield a purer solution of the
more volatile component.[22]
In reality, each cycle at a given temperature does not occur at
exactly the same position in the fractionating column;
theoretical plate is thus a concept rather than an accurate
description.

More theoretical plates lead to better separations. A
spinning band distillation system uses a spinning band of
Teflon or metal to force the rising vapors into close
contact with the descending condensate, increasing the number of
theoretical plates.[23]

Steam distillation

Like
vacuum distillation, steam distillation is a method
for distilling compounds which are heat-sensitive. This process
involves using bubbling steam through a heated mixture of the
raw material. By Raoult's law, some of the target compound will
vaporize (in accordance with its partial pressure). The vapor
mixture is cooled and condensed, usually yielding a layer of oil
and a layer of water.

Vacuum distillation

Some compounds have very high boiling points. To boil such
compounds, it is often better to lower the pressure at which
such compounds are boiled instead of increasing the temperature.
Once the pressure is lowered to the vapor pressure of the
compound (at the given temperature), boiling and the rest of the
distillation process can commence. This technique is referred to
as vacuum distillation and it is commonly found in the
laboratory in the form of the
rotary evaporator.

This technique is also very useful for compounds which boil
beyond their
decomposition temperature at atmospheric pressure and which
would therefore be decomposed by any attempt to boil them under
atmospheric pressure.

Air-sensitive vacuum distillation

Some compounds have high boiling points as well as being
air sensitive. A simple vacuum distillation system as
exemplified above can be used, whereby the vacuum is replaced
with an inert gas after the distillation is complete. However,
this is a less satisfactory system if one desires to collect
fractions under a reduced pressure. To do this a "pig" adaptor
can be added to the end of the condenser, or for better results
or for very air sensitive compounds a
Perkin triangle apparatus can be used.

The Perkin triangle, has means via a series of glass or
Teflon taps to allows fractions to be isolated from the rest
of the
still,
without the main body of the distillation being removed from
either the vacuum or heat source, and thus can remain in a state
of
reflux. To do this, the sample is first isolated from the
vacuum by means of the taps, the vacuum over the sample is then
replaced with an inert gas (such as
nitrogen or
argon)
and can then be stoppered and removed. A fresh collection vessel
can then be added to the system, evacuated and linked back into
the distillation system via the taps to collect a second
fraction, and so on, until all fractions have been collected.

Short path distillation is a distillation technique
that involves the distillate traveling a short distance, often
only a few
centimeters. A classic example would be a distillation
involving the distillate traveling from one glass bulb to
another, without the need for a condenser separating the two
chambers. This technique is often used for compounds which are
unstable at high temperatures. The Advantage is that the heating
temperature can be considerably lower (at this reduced pressure)
than the boiling point of the liquid at standard pressure, and
that the distillate only has to travel a short distance before
condensing. A
Kugelrohr apparatus can be used for Short path distillation.

Other types

In a
kugelrohr a short path distillation apparatus is
typically used (generally in combination with a (high)
vacuum) to distill high boiling (> 300 °C) compounds. The
apparatus consists of an oven in which the compound to be
distilled is placed, a receiving portion which is outside of
the oven, and a means of rotating the sample. The vacuum is
normally generated by using a high vacuum pump.

The process of
reactive distillation involves using the reaction vessel
as the still. In this process, the product is usually
significantly lower-boiling than its reactants. As the
product is formed from the reactants, it is vaporized and
removed from the reaction mixture. This technique is an
example of a continuous vs. a batch process; advantages
include less downtime to charge the reaction vessel with
starting material, and less workup.

Destructive distillation involves the strong heating of
solids (often organic material) in the absence of oxygen (to
prevent combustion) to evaporate various high-boiling
liquids, as well as
thermolysis products. The gases evolved are cooled and
condensed as in normal distillation. The destructive
distillation of
wood to give
methanol is the root of its common name - wood
alcohol.

Pervaporation is a method for the separation of mixtures
of liquids by partial vaporization through a non-porous
membrane.

Dry distillation, despite its name, is not truly
distillation, but rather a chemical reaction known as
pyrolysis in which solid substances are heated in a
strongly
reducing atmosphere and any volatile fractions are
collected.

Extractive distillation is defined as distillation in
the presence of a miscible, high boiling, relatively
non-volatile component, the solvent, that forms no azeotrope
with the other components in the mixture.

Flash evaporation (or partial evaporation) is the
partial vaporization that occurs when a saturated liquid
stream undergoes a reduction in pressure by passing through
a throttling
valve or other throttling device. This process is one of
the simplest unit operations.

Azeotropic distillation

Interactions between the components of the solution create
properties unique to the solution, as most processes entail
nonideal mixtures, where
Raoult's law does not hold. Such interactions can result in
a constant-boiling
azeotrope which behaves as if it were a pure compound
(i.e., boils at a single temperature instead of a range). At an
azeotrope, the solution contains the given component in the same
proportion as the vapor, so that evaporation does not change the
purity, and distillation does not effect separation. For
example,
ethyl alcohol and
water form an azeotrope of 95.6% at 78.1 °C.

If the azeotrope is not considered sufficiently pure for use,
there exist some techniques to break the azeotrope to give a
pure distillate. This set of techniques are known as
azeotropic distillation. Some techniques achieve this by
"jumping" over the azeotropic composition (by adding an
additional component to create a new azeotrope, or by varying
the pressure). Others work by chemically or physically remove or
sequester the impurity. For example, to purify ethanol beyond
95%, a drying agent or a
desiccant such as
potassium carbonate can be added to convert the soluble
water into insoluble
water of crystallization.
Molecular sieves are often used for this purpose as well.

Immiscible liquids, such as water and toluene, easily form
azeotropes. Commonly, these azeotropes are referred to as a low
boiling azeotrope because the boiling point of the azeotrope is
lower than the boiling point of either pure component. The
temperature and composition of the azeotrope is easily predicted
from the vapor pressure of the pure components, without use of
Raoult's law. The azeotrope is easily broken in a distillation
set-up by using a liquid-liquid separator ( a decanter ) to
separate the two liquid layers that are condensed overhead. Only
one of the two liquid layers is refluxed to the distillation
set-up.

High boiling azeotropes, such as a 20 weight percent mixture
of hydrochloric acid in water also exist. As implied by the
name, the boiling point of the azeotrope is greater than the
boiling point of either pure component.

To break azeotropic distillations and cross distillation
boundaries, such as in the DeRosier Problem, it is necessary to
increase the composition of the light key in the distillate.

Breaking an azeotrope with
unidirectional pressure manipulation

A vacuum distillation can be used to "break" an azeotropic
mixture. Varying the temperature of the vapour generating flask
when distilling an azeotrope from cold to the solutions boiling
point does not produce a continuously sliding ratio of product
to contaminate in the distillate. The two separate boiling
points still remain, they merely overlap; these can be thought
of as required activation energies for the release of a
particular vapour. By exposing an azeotrope to a vacuum, it's
possible to bias the boiling point of one away from the other by
exploiting the difference between each components vapour
pressure. When the bias is great enough, the two boiling points
no longer overlap and so the azeotropic band disappears.

This method is not without drawbacks. As an example, exposing
a solution of water and ethanol to a 70 torr vacuum will allow
for absolute ethanol to be distilled. However, due to the low
pressure atmosphere, the ethanol vapour requires a significantly
cooler condenser surface to liquefy, going from 78.3 °C at
atmospheric pressure to 24.5 °C at 70 torr; failure to provide
such results in the vapours passing through the condenser and
into the vacuum source. This can also affect the efficiency of
the condenser, as the liquefying temperature drops towards the
minimum the condensing equipment can cool to, the thermal
gradient across the liquefying surfaces reduces and, so with it,
the rate at which heat can be extracted from the vapour.

Conversely, increasing a distillation pressure can also break
an azeotrope, but will bring with it the possibility of thermal
decomposition, for organic compounds in particular, and may be
more beneficial to high temperature tolerant distillations, such
as those of the metallic salts.

Pressure-swing Distillation

This method of distillation can be used to separate
azeotropic mixtures and relies on a principle similar to vacuum
distillation, that being the manipulation of boiling points by
altering the pressure of the atmosphere to which a solution is
exposed.

It might be chosen over pure vacuum distillation of an
azeotrope if that solution, for instance, had such a low
liquefying point at the pressure required to break the azeotrope
that the equipment was unable to provide for it, allowing the
product to stream out of the condenser and into the vacuum
source. Here, rather than manipulate just one boiling point, one
or more are altered, one after the other; with the number of
pressure alternations being determined by the number of
components in the feed solution considered to be contaminants.
This could be beneficial to a purification as it is likely to
create less extreme thermal requirements. Simply, instead of
swinging distillation pressure in one direction alone in an
attempt to break the azeotrope in one step, the break is
performed in two or more steps with pressure swung in two
directions to create an operating band centered around more
accessible temperatures; perhaps going from a negative pressure
to atmospheric and on to a positive pressure. In essence,
pressure-swing distillation is an attempt to reduce extreme
conditions by dispersing the manipulation load across the
equipment generating the distillation environment.

If a continuous feed is desired, or the distillation
pressures required are extreme enough to warrant specialised
design, each step may require a physically separate column. If
only a batch run is required and the same column can perform
under all the required pressures, this single column may
suffice; with the vapour generating flask being emptied after
the first distillation, the first distillate run back to the
start and the distillation rerun under the second pressure
conditions, and so on.

Selection of which component the distillate should be biased
towards may be made based on the energy required to evaporate it
from the feed solution.

Pressure-swing distillation is employed during the
purification of
ethyl acetate after its catalytic synthesis from ethanol.

Industrial distillation[24][25]
is typically performed in large, vertical cylindrical columns
known as distillation towers or distillation columns
with diameters ranging from about 65 centimeters to 16 meters
and heights ranging from about 6 meters to 90 meters or more.
When the process feed has a diverse composition, as in
distilling
crude oil, liquid outlets at intervals up the column allow
for the withdrawal of different fractions or products
having different
boiling points or boiling ranges. The "lightest" products
(those with the lowest boiling point) exit from the top of the
columns and the "heaviest" products (those with the highest
boiling point) exit from the bottom of the column and are often
called the bottoms.

Diagram of a typical industrial distillation tower

Large-scale industrial towers use
reflux to achieve a more complete separation of products.
Reflux refers to the portion of the condensed overhead liquid
product from a distillation or fractionation tower that is
returned to the upper part of the tower as shown in the
schematic diagram of a typical, large-scale industrial
distillation tower. Inside the tower, the downflowing reflux
liquid provides cooling and condensation of the upflowing vapors
thereby increasing the efficacy of the distillation tower. The
more reflux is provided for a given number of
theoretical plates, the better is the tower's separation of
lower boiling materials from higher boiling materials.
Alternatively, the more reflux is provided for a given desired
separation, the fewer theoretical plates are required.

Section of an industrial distillation tower showing
detail of trays with bubble caps

Design and operation of a distillation tower depends on the
feed and desired products. Given a simple, binary component
feed, analytical methods such as the
McCabe-Thiele method[25][26]
or the
Fenske equation[25]
can be used. For a multi-component feed,
simulation models are used both for design and operation.
Moreover, the efficiencies of the vapor-liquid contact devices
(referred to as "plates" or "trays") used in distillation towers
are typically lower than that of a theoretical 100% efficient
equilibrium stage. Hence, a distillation tower needs more
trays than the number of theoretical vapor-liquid equilibrium
stages.

In industrial uses, sometimes a packing material is used in
the column instead of trays, especially when low pressure drops
across the column are required, as when operating under vacuum.

This packing material can either be random dumped packing
(1-3" wide) such as
Raschig rings or
structured sheet metal. Liquids tend to wet the surface of
the packing and the vapors pass across this wetted surface,
where
mass transfer takes place. Unlike conventional tray
distillation in which every tray represents a separate point of
vapor-liquid equilibrium, the vapor-liquid equilibrium curve in
a packed column is continuous. However, when modeling packed
columns, it is useful to compute a number of "theoretical
stages" to denote the separation efficiency of the packed column
with respect to more traditional trays. Differently shaped
packings have different surface areas and void space between
packings. Both of these factors affect packing performance.

Another factor in addition to the packing shape and surface
area that affects the performance of random or structured
packing is the liquid and vapor distribution entering the packed
bed. The number of
theoretical stages required to make a given separation is
calculated using a specific vapor to liquid ratio. If the liquid
and vapor are not evenly distributed across the superficial
tower area as it enters the packed bed, the liquid to vapor
ratio will not be correct in the packed bed and the required
separation will not be achieved. The packing will appear to not
be working properly. The
height equivalent of a theoretical plate (HETP) will be
greater than expected. The problem is not the packing itself but
the mal-distribution of the fluids entering the packed bed.
Liquid mal-distribution is more frequently the problem than
vapor. The design of the liquid distributors used to introduce
the feed and reflux to a packed bed is critical to making the
packing perform to it maximum efficiency. Methods of evaluating
the effectiveness of a liquid distributor to evenly distribute
the liquid entering a packed bed can be found in references.[28][29]
Considerable work as been done on this topic by Fractionation
Research, Inc. (commonly known as FRI).[30]

Distillation in food processing

Distilled beverages

Carbohydrate-containing plant materials are allowed to
ferment, producing a dilute solution of
ethanol in the process. Spirits such as
whiskey and
rum
are prepared by distilling these dilute solutions of ethanol.
Other components than ethanol are collected in the condensate,
including water, esters, and other alcohols which account for
the flavor of the beverage.

Distillation using semi-microscale apparatus. The
jointless design eliminates the need to fit pieces
together. The pear-shaped flask allows the last drop of
residue to be removed, compared with a similarly-sized
round-bottom flask The small holdup volume prevents
losses. A pig is used to channel the various distillates
into three receiving flasks. If necessary the
distillation can be carried out under vacuum using the
vacuum adapter at the pig.